The goal in the production of silver-cadmium oxide electrical contacts is to create a fully dense material having a homogenous distribution of rounded cadmium oxide particles tightly embedded in the silver matrix. Methods of attaining such an ideal structure have been sought, but no process has as yet achieved this ideal condition.
Three methods are currently used commercially to produce silver cadmium oxide electrical contacts. The simplest and least costly process is the conventional powder metallurgy method wherein fine powders of silver and cadmium oxide are blended together to form a uniform admixture. The powders after blending are compacted under pressure to achieve a green unsintered contact. The contact is sintered at a temperature below the melting point of silver and thereafter coined to achieve near theoretical density. The degree of dispersion of the cadmium oxide in the silver matrix is primarily dependent upon the mixing achieved. While this method is relatively simple, uniform and small cadmium oxide particles are not attained. The particles of cadmium oxide are irregularly shaped and have a relatively wide particle size range. For example, the range is generally from 1 to 5 microns when the average is 2 microns.
Another process involves starting with silver and cadmium metals, forming finely divided powders of these two metals then oxidizing the cadmium oxide. U.S. Pat. No. 3,954,459 gives a description of such a process. In the process of that patent a fraction of the total powders in the 0.2 mm to 1 mm diameter range is separated and oxidized at temperatures of about 800.degree. C. The oxidized powder is further comminuted to produce fragments having a particle size range in the 200 to 300 micron range. The comminuted particles are compacted using pressures of from about 900 K/cm.sup.2 to about 720 K/cm.sup.2. Zinc stearate or stearic acid is used as a molding aid and sintering is done in air at about 800.degree. C. for about 1 hour. To achieve a pore-free contact post-sintering compaction is also done, preferably by hot-pressing at a pressure of about 7200 K/cm.sup.2 and at a temperature of about 650.degree. C. While the foregoing process produces a more uniform distribution of cadmium oxide than is achieved by blending silver and cadmium oxide as previously described, migration of cadmium during oxidation leads to localized areas which are cadmium oxide-rich. There is no direct control over the particle size of cadmium oxide thus the particle size of cadmium oxide in the contact varies over a relatively large range.
A third more recent process is to uniformly blend finely divided silver and finely divided cadmium oxide, reduce the cadmium oxide at a temperature above the melting point of cadmium (above 321.degree. C.), oxidize the alloy powder, and thereafter compact and sinter the contact as in the other processes. Distribution of cadmium oxide is improved over the other two previously described processes. However, the densities of the compacts after sintering are seldom above about 93% of the theorectical density. In addition, the cadmium oxide particles range from less than 1 micron to larger than 5 microns and are not spherical in shape.
It is believed, therefore, that the production of silver cadmium oxide compacts that after sintering have a density of greater than 96% of the theoretical density and contain a very homogenous distribution of rounded cadmium oxide particles with a large percentage in the 3 to 5 micron range would be an advancement in the art.